In the past, various techniques were utilized to adhesively bond the individual laminations of a stator core together in an axially stacked relation against separation. For the most part, bonded stator cores and an associated rotor were employed in hermetic motors for driving refrigerant or hermetic compressor assemblies. The bonded core was provided with a plurality of bolt holes therethrough for receiving bolts adapted to mount the bonded stator in cantilever fashion to a stationary frame of the compressor assembly. The rotor was rotatably disposed within a bore of the bonded stator core, and the rotor was also mounted in cantilever fashion on a shaft therefor which was connected so as to drive the compressor assembly.
U.S. Pat. No. 3,490,143 and U.S. Pat. No. Re. 26,788 issued to Bobbie B. Hull are representative examples of the prior art concerning bonded stator cores. These prior art patents teach the desirability of generally isolating adhesive bonding material from bolt compression regions of the bonded stator core between its next adjacent laminations immediately adjacent the bolt holes through the bonded stator core. The isolation of the adhesive bonding material from these bolt compression regions is believed to be necessary since such regions are affected by compressive forces exerted thereon by the torqued-down mounting bolts when the bonded stator core is mounted in the cantilevered fashion onto the stationary frame of the compressor assembly, as discussed above. For instance, if adhesive bonding material was present in the aforementioned bolt compression regions during normal usage of the bonded stator core in the compressor assembly, thermoplastic flow of the adhesive bonding material away from such regions might possibly occur and may cause a loss of mounting bolt torque thereby to result in relative slippage of the individual laminations of the bonded stator core. This relative slippage of the individual laminations is, of course, believed to be a disadvantageous or undesirable feature which may result in misalignment between the bore of the bonded stator core and the rotor associated therewith causing degradation of the motor performance or perhaps the failure of the motor. Bonded stator cores which exhibit the above discussed disadvantageous feature are referred to as having a high compressibility characteristic.
In an effort to overcome the above discussed high compressibility characteristic present in some of the prior art bonded stator cores, various manufacturing techniques have been proposed for applying adhesive bonding material to the edges of lamination stacks of a stator core in such a way as to keep the adhesive bonding material from migrating into the bolt compression regions subjected to the compressive mounting forces, as previously mentioned. These manufacture techniques include: exerting greater than normal bolt-down clamping forces on the lamination stack of the stator core prior to application thereto of the adhesive bonding material to minimize migration of the adhesive bonding material into the bolt compression regions; applying similar higher bolt-down clamping forces after the application of the adhesive bonding material but before hardening thereof so as to squeeze the adhesive bonding material out of the bolt compression regions; or applying controlled, metered amounts of the adhesive bonding material to the lamination stack with such amount being insufficient to effect migration of the adhesive bonding material fully into the bolt compression regions.
Typically, in these prior art manufacturing techniques or processes, a liquid adhesive bonding material was employed having a solvent carrier and a solids content of a relatively low percentage. One suggested reason for using a liquid adhesive bonding material having a low solids content-solvent carrier is believed to be that it enhanced the capillary action which effects the penetration of such adhesive bonding material into the interlaminar surfaces of the stator core. A representative teaching to this effect is found in U.S. Pat. No. 4,085,347 which discloses the use of a carefully controlled or metered amount of adhesive bonding material to keep such material away from the bolt compression regions of the stator core. This reference teaches that while shear strength of the stator core tends to increase as the solids content of the liquid adhesive bonding material increases, the capillary migration is reduced accordingly; and, the reference also teaches that a liquid adhesive bonding material be used having a concentration of solids in the range of about 20%-30% by weight to provide uniform distribution, rapid migration, and adequate shear strength per unit of bonding area. This prior art patent emphasizes the desirability of covering a substantial percentage of the interlaminar surface area, at least 50% of the area between adjacent bolt holes up to as much as all the surface area except for the bolt compression regions. Another exemplary prior art U.S. Pat. No. 3,573,129, while not referring to specific solids contents, teaches the use of epoxy-type varnishes of syrupy consistency and further teaches that the capillary action during the "wicking" step can be facilitated by creating a temperature differential between the liquid adhesive bonding material and the lamination stack of the stator core giving, as one example, an adhesive at room temperature with the stack warmed to between 200.degree. F. and 400.degree. F. (93.degree. C.-204.degree. C.). It may be noted that this prior art patent disclosure combines the use of a carefully controlled or metered amount of liquid adhesive bonding material followed by application of high compressive forces after the "wicking" step to squeeze the liquid adhesive bonding material away from the bolt compression regions.
Thus the prior art, in the manufacture of bonded stator cores, has taught the enhancement of capillary action as a means of applying as much adhesive bonding material as possible to the interlaminar surfaces of the stator core to attain necessary shear strength properties therefor while at the same time avoiding the introduction or retention of adhesive bonding material in the bolt compression regions of the stator core so as to attain a desired low compressibility characteristic. This above discussed dichotomy with respect to adhesive application criteria has resulted in rather complex and intricate manufacturing apparatus and processes, and it is believed that such apparatus and processes have not proven entirely satisfactory for high volume-low cost production of bonded stator cores which possess the desired properties of adequate shear strength and low compressibility characteristics while at the same time having a desired degree of axial alignment and end face squareness.
In presently known adhesive bonding techniques or manufacturing processes, the bolt compression regions of the stator core is subjected to compressive forces that equal and sometimes exceed the mounting forces encountered in the field during normal use. It is believed that the need for this step introduces disadvantageous or undesirable features or complexities in the manufacturing process, which may be prudent to avoid. It is also believed that employing this step may have a tendency to produce stator cores which are not as uniformly aligned and squared as may be desired by the end user.
In addition, while many of the prior art manufacturing techniques purport to lend themselves to a high volume production line, it is believed that such techniques often may require excessive handling of the lamination stacks during various aspects of the alignment, bonding and hardening phases of the stator core manufacture. Also, in at least some of these prior art techniques, it is believed that relatively long times may be required to effect the application of the bonding material and for the ensuing hardening thereof.